xco uses the database of standard X11 named colors to display
name-color pairs.  Appearance is based on the X10 xcolors program.  To
change the foreground color, click the left mouse button in the color
window you want.  To change the background color, click the right
mouse button in the color window you want.  Clicking the middle mouse
button sets the colors back to the original foreground and background.
You can switch quickly through colors by holding down the left or
right button and sliding the cursor through the colored windows.  More
colors can be seen by clicking on the arrows at the bottom of the
screen.
